Retroreflective sheeting is known in the prior art. Such sheeting includes an array of independently retroreflecting optical elements. The elements may be spheres or prisms. Mutually contiguous pyramidal prisms can be molded or embossed on one side of a transparent sheet material usually formed from plastic. The prisms have three faces which intersect each other at 90° angles. Such pyramid-shaped prisms are known in the art as cube corners. Cube corner prisms, uncoated with a reflective metal layer, possess the property of total internal reflection (TIR) for incident rays of light that impinge the surfaces of the cube corner within a certain critical range of angles. Consequently, a substantial portion of a beam of light directed at such a cube corner array will be retroreflected back in the direction of its source if it impinges the array within the critical range of angles. Alternatively, the cube corner prism faces can be coated with a reflective metal layer to be effective for more incident rays.
Retroreflective sheet material is used to enhance the nighttime visibility of street and highway signage, as well as trucks, buses and semi-tractor trailers that often travel along interstate highways. In the latter application, strips of retroreflective material are typically adhered around the edges of the trailer so that other motorists in the vicinity of the vehicle will readily perceive not only the presence but also the extent of the vehicle. The use of such retroreflective strips on trucks and other vehicles has doubtlessly contributed to the safety of the highways by preventing nighttime collisions which would otherwise have occurred.
Road signs also use retroreflective sheet material, but the requirements differ in two important ways. Truck stripes have no need to be conspicuously light by day, as road signs do. Truck stripes must be able to retroreflect light at very large angles of incidence, which few road signs must do.
Metallized cube corner sheeting generally performs better under nighttime conditions than unmetallized cube corner sheeting. Unmetallized cube corners rely entirely on TIR for reflection. Consequently, light which impinges on the prism faces at an angle greater than the critical angle will be reflected only feebly. For example, for retroreflective sheeting formed from a transparent material having an index of refraction of n =1.5 the critical angle is about 41.82°. Light incident at a face at 41° loses 62% of its intensity. A cube corner prism with unmetallized faces may have one or two faces failing TIR for a particular incoming illumination. By contrast, when the cube corners of a retroreflective sheeting are metallized by the application of a thin coating of a specular material such as aluminum or silver, a substantial percentage of incident light will always be retroreflected regardless of the angle of incidence. Even though the total effective reflectance is only about 61% after the incident light impinges all three of the cube corner surfaces necessary for retoreflectivity, considering all incidence and orientation angles, the net rating for retroreflection is generally higher with metallized vs. unmetallized cube corners. Another advantage of metallized versus unmetallized cube corners is that metallized cube corners do not require a hermetic seal of the surface of the sheet material where the cube corners are molded or embossed. Such hermetic seals sacrifice about 25-30% of the total sheeting area, rendering unmetallized sheeting dimmer than it otherwise would be when exposed to a beam of an automobile headlight. In short, the higher retroreflectance of metallized versus unmetallized retroreflective sheeting over a broader range of incidence angles in combination with the non-necessity of hermetic seals renders it generally brighter under nighttime conditions.
Unfortunately, presently-known forms of metallized cube corner sheeting perform poorly under daytime conditions. This is a major shortcoming, as transportation regulations in the United States, Europe, China and Brazil require such sheeting to have a minimal daytime luminance factor as measured by a 0/45 or 45/0 calorimeter. In contrast to non-metallized cube corner sheeting material, metallized sheeting can appear relatively dark under daytime conditions. This deficiency in daytime luminance has resulted either in the use of non-metallized cube corner sheet materials which are inherently more limited in their nighttime performance or in the use of metallized cube corner sheet materials having white marks printed over some fraction of the of the surface, or white spaces between prisms, which enhance daytime luminance but degrade nighttime performance.
Clearly, there is a need for a metallized cube corner retroreflective sheeting which maintains all of the nighttime performance advantages of such sheeting, but which also complies with the daytime luminance standards mandated by transportation regulations. Ideally, such sheet material should be relatively easy and inexpensive to manufacture and mount on a vehicle.